1. Field of the Invention
The present invention relates generally to a heat transfer apparatus and method, and more particularly to an improved apparatus and method for transferring heat from an externally disposed heat transfer element through a retarding heat transfer material to a pipe.
2. Background
The operation of pumping a fluid through a process pipeline frequently requires that the process pipeline be heated to prevent the fluid from condensing or becoming too viscous to pump through the pipeline. Heating and maintaining the proper temperature of the process pipeline, which aids in regulating the flow of the process fluid, is often accomplished by positioning an external heat transfer element, or tracer line, on the process pipe.
There are two types of external heat transfer elements that are generally used in these situations: (1) an electric heating cable, or (2) a small pipe or tube which is heated by a hot fluid, typically steam, passing therethrough.
Each type of external tracer line (electric cable or steam tubing) can be installed on the process pipe by either of two methods: (1) the convection tracing method, or (2) the conduction tracing method.
Convection tracing consists of attaching the small steam tube or the electric heating cable directly to the external surface of the process pipe. The steam tube, typically small diameter copper or stainless steel tubing, or the electric heating cable may be either spirally wrapped around or run parallel to the process pipe. Convection tracing relies on heat transfer by "spot" contact between the tracer line and the process pipe together with convection (thus termed "convection tracing") to the surrounding air and radiation from the tracer line to the process pipe. The heat transfer characteristics of the convection tracing method are not only low but also unpredictable. The unpredictability of the heat transfer characteristics precludes precise or even reasonable control of the temperature of the process fluid.
In conduction tracing, an external tracer line is attached to the process pipe with a heat transfer material. The primary goal of conduction tracing has been to provide increased heat transfer from the heat transfer element (or tracer line) to the process pipe. The prior art heat transfer materials, such as the cements or putties disclosed in U.S. Pat. No. 3,331,946 and U.S. Pat. No. Re. 29,332, have properties which include a high thermal conductivity, thus, a relatively high heat transfer coefficient, along with good adhesion to metal under varying temperature conditions. Heat transfer cements effect heat transfer by conduction and increase the usable heat output from the tracer line to the process pipe by approximately 1100%. An external steam tracer line bonded with a heat transfer cement is efficient to use from a heat loss standpoint and economical to install.
U.S. Pat. No. 3,331,946 discloses a heat transfer apparatus wherein a heat transfer material or cement is placed in a channel member in a putty-like or plastic state. The channel member with heat transfer material is then pressed downwardly on top of the heat transfer element (a steam tube or electric heating cable) which embeds the heat transfer element in the heat transfer material. U.S. Pat. Nos. 4,123,837 and 4,203,186 disclose methods for applying a heat transfer cement or putty disposed in a channel member to a heat transfer element. U.S. Pat. No. Re. 29,332 discloses a heat transfer assembly in which a flexible and resilient heat transfer material is pre-shaped to closely conform with a heat transfer element and to conform to the internal area of a channel member which is applied over the heat transfer material and heat transfer element at the time of installation. The heat transfer material is extruded or molded under enough pressure and force to virtually eliminate any air pockets or bubbles from the heat transfer material strip.
The primary objective of all of the prior art heat transfer materials has been to increase the amount of heat transferred from the tracer line to the process pipe as compared to a bare tracer line attached to a process pipe. Thus, all the prior art heat transfer materials have a high rate of thermal conductivity. U.S. Pat. No. 3,972,821 states that synthetic resins and plastics used in heat transfer materials have a relatively low thermal conductivity, generally below 10 BTU-In/Hr-Ft.sup.2 -.degree.F., so the heat transfer material will have dispersed in the synthetic resins and plastics a substantial portion of particulate solid having a significant thermal conductivity of at least 100 BTU-In/Hr-Ft.sup.2 -.degree.F.
However, the situation often arises in Which it is desirable to use a relatively high temperature heat medium, such as steam, in a heat tracing system to heat and maintain a low temperature process pipe without overheating the process pipe. For example, high temperature steam is oftentimes commonly available for use as the heating medium in an external tracer line; however, prior art heat tracing systems would deliver too much heat for many temperature sensitive applications. There are many process fluids which are very temperature sensitive. A few temperature sensitive process fluids include products such as caustic soda, amines, various acids, etc.
In other situations it may be uneconomical to use a high temperature heat transfer element where less heat would suffice. For example, the use of a steam tracer line with a process pipe which only requires freeze protection will generally heat the process pipe more than necessary for simple freeze protection when attached by prior art methods. This is inefficient and wastes an excessive amount of heat in the heat tracing system.
Similarly, today many heat traced process pipes are designed with more insulation around the process pipe and the heat tracing system, for greater energy efficiency, than in the past. A by-product of increased insulation is that a heat transfer element which formerly would have required the use of a heat transfer material to increase heat conductance to the process pipe now produces too much heat for the more heavily insulated pipe and tracing system.
A current industry practice to mitigate these problems is to reduce heat transfer from the tracer line to the process pipe by inserting spacer blocks between the heat transfer element and the process pipe. This technique has several drawbacks. While this may reduce overall heat transfer to the process pipe, it is very difficult to achieve uniform heat transfer in this manner due to the difficulty of maintaining uniform spacing of the heat transfer element from the process pipe. Overheating or inadequate heating of the process pipe is often experienced. The rate of heat transfer by this technique is very sensitive to the distance between the heat transfer element and the process pipe. This has forced plant engineers to design most of their temperature sensitive process pipelines around electric heat tracing systems which can be accurately controlled, albeit at somewhat higher costs.
It would be advantageous to have a retarding heat tracing system adapted to utilize a high temperature heat transfer element to heat and uniformly maintain a low temperature process pipe without overheating the process pipe. It would also be advantageous to have a retarding heat transfer system which optimizes the heat output of the retarding heat transfer system to the process pipe while minimizing the amount of wasted heat given off by the retarding heat transfer system. Additionally, it would be advantageous to have a heat transfer material in a heat transfer system which can be formed into a heat transfer material strip having a required thermal conductivity which has been calculated for the specific application.